ABSTRACT Usher syndrome (USH) is an inherited disorder responsible for more than half of combined deafness and blindness in humans. It affects both photoreceptors and cochlear hair cells, the sensory neurons of the retina and inner ear, respectively. Usher syndrome type III (USH3) is caused by mutations in clarin-1 (CLRN1) gene, leading to the gradual and permanent loss of both vision and normal hearing. Cochlear implantation alleviates to some degree the hearing impairment, but there is no treatment that prevents the progressive photoreceptor cell death that leads to blindness. Our goal is to develop an understanding of CLRN1 function in the retina so that a rational gene therapy approach for USH3 patients can be developed. Efforts to identify efficient therapeutic strategies for vision loss are hindered by the lack of USH3 animal models that mimic the human retinal disorder, and by conflicting reports on CLRN1?s endogenous cellular localization, with only one study showing its presence in photoreceptors. All current USH3 mouse models display a rapid hearing loss but normal retinal function and morphology. As part of a collaborative effort, we showed that CLRN1 gene replacement therapy using adeno-associated viral vectors (AAVs) can prevent hearing loss for at least 1 year following cochlear delivery in a mouse model of USH3, the first successful proof of concept experiment for this disorder. We also showed that retinal function is preserved following a similar gene therapy approach to treat the retina in a new mouse model of USH3, which displays a slow, progressive loss of retinal function with aging. These results suggest that postnatal CLRN1 delivery can prevent further loss of function in sensory cells, a particularly relevant issue for patients with more advanced stages of disease. Our central hypothesis is that CLRN1 plays an essential role in photoreceptor cells, structurally stabilizing the fragile connecting cilium region by interacting with other USH proteins and cytoskeletal elements. We address this hypothesis with three specific aims: 1) identify CLRN1?s endogenous retinal localization and the proteins that interact with CLRN1 in the mouse retina; 2) determine the retinal expression pattern and pathogenic effects of mutant CLRN1 proteins in-vivo; 3) test several gene therapy options in USH3 mouse models, targeting either the entire retina, or the photoreceptors alone. Completion of these goals would firmly identify CLRN1?s general function and, critically, serve to pinpoint which cells would need to be targeted for USH3 gene replacement therapy to prevent the loss of vision in patients.